The invention relates to a method and to an apparatus for determining the object distance between an opto-electronic sensor working according to the principle of triangulation and a sensed object.
With known sensors which work according to the principle of triangulation, an emitted light spot is imaged on the object whose distance is to be determined and from the object onto a spatially resolving receiver. The position of the reflected and/or remitted light spot on the receiver is dependent on the distance between the sensor and the object, also known as the operating distance. The position of the center of the light spot on the receiver can thus be used as a measure for the distance to be determined. It is known for this purpose to divide the light-sensitive range of the receiver into two sub-ranges, namely a near range and a far range. The distribution of the intensity of the imaged light spot between the near range and the far range is dependent on the object distance so that the difference between the output signals of the two ranges forms the measure for the object distance.
The disadvantage with these sensors is that interference signals superimposed on the actual received signal which comes from the light spot reflected and/or remitted by the object cannot be recognized as such. Sources of such interference signals are, for example, defects in or contaminations of the sensor optical system, reflecting or shiny areas or areas of high contrast either on the object whose distance is to be determined or on interfering objects arranged to the side or the rear of the object to be sensed and which are also known as background objects.
It is an object of the invention to determine the distance between the sensor and an object in the simplest and most reliable manner possible using an opto-electronic sensor working according to the principle of triangulation independently of any error sources which may be present and which adulterate the actual received signal.
This object is satisfied, on the one hand, in that the sensor has at least one measuring channel between a transmission unit for the transmitting of electromagnetic sensing rays into the measuring region and a reception unit for the detection of sensing rays reflected and/or remitted from the measuring region, in that the sensor has at least one additional channel having a further transmission unit and/or a further reception unit in addition to the transmission unit and the reception unit of the measuring channel, and in that the received signals of the measuring channel and of the additional channel are jointly evaluated in order to determine the object distance.
In accordance with the invention, additional information is made available by the additional channel and can be used, by the joint evaluation of the information from the measuring channel, in order to identify error sources as such and so to reduce the influence of the error sources on the distance measurement.
The joint evaluation takes place in an evaluation unit which is associated with the sensor and to which the reception unit or reception units are connected. Suitable mathematical evaluation methods, for example cross-correlations between a stored or learned intensity distribution of the received signals and a current intensity distribution, can be used respectively in dependence on the design of the method for the distance measurement or on the design and the mode of operation of the sensor.
It is preferred for the measuring channel and the additional channel to be operated jointly and in particular at least substantially simultaneously for each object distance.
This also includes the case where the channels are successively controlled in fast succession by, for example, a plurality of transmission units successively emitting their sensing rays in short time intervals and/or a plurality of reception units being successively read out in order to determine the respective object distance by a joint evaluation of the received signals.
In accordance with an embodiment of the invention, provision can be made that only the received signal of the measuring channel is used for the determination of a distance value, that a determination is made by means of the received signal of the additional channel whether the received signals meet at least one criterion and that, if the additional criterion is met, the distance value is used as a measure for the object distance.
Only the received signal of the measuring channel is thus used for the actual distance measurement after a check has been made with the aid of the received signal of the additional channel whether the measurement satisfies certain demands and the received signals meet certain additional conditions. A decision can thus be made by means of the additional channel whether the received signal of the measuring channel has been adulterated or not.
In accordance with an alternative embodiment of the invention, it can be provided that the received signals of both the measuring channel and the additional channel are used to determine a distance value serving as a measurement for the object distance.
In this respect, the distance value is not only determined with the aid of one of the received signals, but the received signals of both the measuring channel and the additional channel are used for the determination of the object distance.
It is preferred that the respective intensity distribution of the received signal on the reception unit or on the reception units be used for the distance determination in the measuring channel and/or in the additional channel.
In accordance with another aspect of the invention, spatially resolving detectors of generally any desired kind can be provided as reception units. The position of a light spot or bead reflected and/or remitted from the object and information on the circumstances of the light spot reflection and/or remittance can be read off from the detected intensity distributions.
In a preferred embodiment of the invention, a separate transmission unit and preferably a common,reception unit are respectively used for the measuring channel and the additional channel.
In this variant of the invention, work is carried out with at least two reception units which are operated such that the emitted sensing rays are incident at different positions on the object so that a corresponding positional difference is detected on the reception unit.
In this respect, the two transmission units can emit the sensing rays in different directions for the formation of the measuring channel and of the additional channel. Then mutually corresponding characteristic ranges of the reception signals of the measuring channel and of the additional channel are each examined, preferably in the joint evaluation, as to whether they are in particular mutually displaced by an expected amount.
Provision is alternatively made, in accordance with a further embodiment of the invention, when a plurality of transmission units are used, that the sensing rays of the measuring channel and of the additional channel are focused at different distances, with preferably a near range being focused at in one channel and a far range being focused at in the other channel.
Then mutually corresponding characteristic ranges of the received signals of the measuring channel and of the additional channel are preferably respectively examined in the joint evaluation as to whether the emitted sensing rays are imaged in particular with different sharpness, in particular different sharpness in accordance with an expected amount.
In a further alternative in accordance with a further embodiment of the invention using a plurality of transmission units, it is provided that the sensing rays of the additional channel are deliberately emitted, in particular in an unfocused, scattered, expanded and/or diffuse manner, such that a spatially expanded sensing zone is emitted into the measuring region. In this respect, the sensing zone can preferably cover a substantial part of the half-space of the sensor on the sensed object side or at least substantially the whole half-space.
Then, in the joint evaluation, a difference is preferably formed between the received signals of the measuring channel and of the additional channel. In this respect, first the received signal of the additional channel is preferably deducted from the received signal of the measuring channel, negative difference values are subsequently set to zero and a resulting positive difference signal is then used for the determination of the distance.
The scattered light resulting in interference signals can be imaged by the spatially expanded sensing zone so that critical viewing regions of the sensor can be monitored for the presence of interfering objects or artifacts.
In accordance with the invention, not only a plurality of transmission units can be provided, but alternatively a division into a plurality of reception units and/or optical reception systems can also be made on the reception side, with a joint transmission unit preferably being used for the measuring channel and the additional channel.
In accordance with a preferred variant of the invention, the transmission unit is in this respect arranged between the at least two reception units and/or optical reception systems, a center and the distance of the center from the position of the transmission unit are respectively determined for the received signals and the mean value of the center distances is used as the measure for the object distance.
In accordance with an alternative variant of the invention, the at least two reception units and/or optical reception systems are arranged at the same side of the transmission unit and the distance between mutually corresponding ranges of the received signals of the measuring channel and of the additional channel are used as the measure for the object distance.
While a plurality of transmission units can be used when reflecting or shiny interfering objects are to be expected next to or behind the object to be sensed, a plurality of reception units and/or optical reception systems are used when errors can be expected as a result of a relatively high-contrast sensed object.
All variants of the invention, the introduction to the description and the following description of the Figures can also be combined with one anotherxe2x80x94provided that they do not contradict one anotherxe2x80x94whereby a particularly secure and reliable determination of the object distance is possible.
The underlying object of the invention is satisfied, on the other hand, by an apparatus for the determination of the object distance between an optoelectronic sensor working according to the principle of triangulation and a sensed object having at least one measuring channel between a transmission unit for the transmission of electromagnetic sensing rays into the measuring region and a reception unit for the detection of sensing rays reflected and/or remitted from the measuring region, having at least one additional channel which has a further transmission unit and/or a further reception unit in addition to the transmission unit and the reception unit of the measuring channel and having an evaluation unit for the joint evaluation of the received signals of the measuring channel and of the additional channel for the determination of the object distance.
All transmission units and reception units are preferably arranged in a common sensor plane which preferably extends perpendicular to the distance direction which corresponds to the shortest distance between the sensor and the object and which is also termed the transmission and/or reception axis.
The or each transmission unit is preferably provided in the form of an LED or of a laser device, for example a laser diode. Furthermore, the or each reception unit is preferably provided in the form of a spatially resolving detector, for example in the form of a single-row or multi-row photodiode array, of a CCD (charge coupled device) or of a PSD (position sensitive device).
Further embodiments of the invention are also set forth in the description and the drawing.
The invention will be described in the following by way of example with reference to the drawing.